d. Although the frame is free to move up and down upon the
uprising shaft, still its inclination to the vertical is determined by
the direction of the line drawn from a buoy in the trough of a wave to
one on the crest. In order to facilitate the free movement, and to
render the rocking effect more accurate and free from vibration, sets
of wheels running on rails fixed to the beam are of considerable
advantage.

The rise and fall of the tides render necessary the adoption of some
such compensating device as that which has been indicated. Of course
it would be possible to provide for utilising the force generated by a
buoy simply moored direct to a ring at the bottom by means of a common
chain cable; but this latter would require to be of a length
sufficient to provide for the highest possible wave on the top of the
highest tide. Then, again, the loose chain at low tide would permit
the buoy to drift abroad within a very considerable area of sea
surface, and in order to take advantage of the rise and fall on each
wave it would be essential to provide at the derrick on the shore end
of the wave-power plant very long toothed bands or equivalent devices
on a similarly enlarged scale.

By providing three or four chains and moorings, meeting in a centre at
the buoy itself but fastened to rings secured to weights at the bottom
at a considerable distance apart, the lateral movement might, no
doubt, be minimised; and for very simple installations this plan,
associated with the device of taking a cable from the buoy and turning
it several times round a drum on shore, could be used to furnish a
convenient source of cheap power. The drum may carry a crank and
shaft, which works the spur-wheel and toothed bands as already
described, so that no matter at what stage in the revolution of the
drum an upward or downward stroke may be stopped, the motion will
still be communicated in a continuous rotary form to the fly-wheel.

But the beam and sliding frame, with buoys, give the best practical
results, especially for large installations. It is in some instances
advisable, especially where the depth of the water at a convenient
distance from the shore is very considerable, not to provide a single
beam reaching the whole distance to the bottom, but to anchor an
air-tight tank below the surface and well beneath the depth at which
wave disturbance is ever felt. From this submerged tank, which
approximately keeps a steady position in all tid